A100H Exploring the Universe: The interaction of light and matter. Martin D. Weinberg UMass Astronomy

A100H Exploring the Universe: The interaction of light and matter Martin D. Weinberg UMass Astronomy astron100h-mdw@courses.umass.edu February 11, 2016 Read: Chap 5 02/11/16 slide 1

Exam #1: Thu 18 Feb Multiple choice and short answers Equations, physical constants will be provided Read: Chap 5 02/11/16 slide 2

Exam #1: Thu 18 Feb Multiple choice and short answers Equations, physical constants will be provided Today: Light and Electromagnetism (LIGHT, Chap. 5) What is the structure of matter? What are the phases of matter? How is energy stored in atoms? What are the three basic types of spectra? How does light tell us what things are made of? Read: Chap 5 02/11/16 slide 2

Properties of Light: review Particle properties Travels in straight lines (bullets) Rays are parallel far from source E = hν Wave properties Rainbow spectrum Interference Light is electromagnetic radiation Wave-Particle Duality: light has both wave-like and particle-like properties! Read: Chap 5 02/11/16 slide 3

Question: Is there an interference pattern if I fired one photon at at time at the double slits? (a) Yes (b) No Read: Chap 5 02/11/16 slide 4

Question: Is there an interference pattern if I fired one photon at at time at the double slits? (a) Yes (b) No Particle-wave duality Read: Chap 5 02/11/16 slide 4

The higher the photon energy... (a) the longer its wavelength. (b) the shorter its wavelength. (c) energy is independent of wavelength. Read: Chap 5 02/11/16 slide 5

What is the structure of matter? Structure of matter [Atom] [Electron cloud] [Nucleus] Read: Chap 5 02/11/16 slide 6

Atomic terminology Atomic Number = # of protons in nucleus Atomic Mass Number = # of protons + neutrons Molecules: consist of two or more atoms (H 2 O,CO 2 ) Read: Chap 5 02/11/16 slide 7

Atomic terminology Isotope: same # of protons but different # of neutrons. ( 4 He, 3 He) Read: Chap 5 02/11/16 slide 8

Phase depends on both temperature and pressure Often more than one phase is present Read: Chap 5 02/11/16 slide 9

Example: familiar phases (of water) Solid (ice) Liquid (water) Gas (water vapor) Phases of same material behave differently because of differences in chemical bonds Read: Chap 5 02/11/16 slide 10

Example: familiar phases (of water) Solid (ice) Liquid (water) Gas (water vapor) Phases of same material behave differently because of differences in chemical bonds Less familiar phase: Ionized (plasma), no chemical bonds Read: Chap 5 02/11/16 slide 10

Ionization: Stripping of electrons, changing atoms into plasma Dissociation: into atoms Breaking of molecules Evaporation: Breaking of flexible chemical bonds, changing liquid into solid Melting: Breaking of rigid chemical bonds, changing solid into liquid Read: Chap 5 02/11/16 slide 11

Thermal or Blackbody Radiation Electromagnetic radiation is caused by moving charges Electrons in a radio antenna Accelerated electrons in an X-ray tube Most material objects are made up of charged particles The temperature of an object proportional to vibration of charges Maxwell Distribution 0.6 0.5 cold Fraction with given speed 0.4 0.3 0.2 0.1 warm [demo] 0 0 1 2 3 4 5 6 7 8 Speed hot Read: Chap 5 02/11/16 slide 12

Thermal or Blackbody Radiation Examples: Hot objects glow! (Fireplace, stove top,... ) Cooler objects like the human body are emitting radiation as well, though mostly at longer wavelengths, in the infrared. Read: Chap 5 02/11/16 slide 13

Thermal or Blackbody Radiation Planck (1900) was trying to understand the nature of radiation that filled an enclosure that had come to equilibrium This emission of radiation that depends on an objects temperature is called thermal or blackbody radiation A perfect blackbody reflects no light (it s only truly black at 0 K) When heated, a blackbody glows with a continuous spectrum which depends only on its temperature Read: Chap 5 02/11/16 slide 14

Thermal or Blackbody Radiation Planck measured the amount of energy in the radiation as a function of wavelength Their results could be displayed as a kind of histogram, with energy plotted against small wavelength intervals (a distribution) Shapes of the curves depended only on the temperature of the enclosure, and not on the material inside This energy distribution became known as the Universal Planck Radiation Law Read: Chap 5 02/11/16 slide 15

Thermal or Blackbody Radiation Distribution for different T Read: Chap 5 02/11/16 slide 16

Thermal or Blackbody Radiation Distribution for the Sun Read: Chap 5 02/11/16 slide 16

Thermal or Blackbody Radiation Wien s Law λ peak = 0.29 T( K) cm = 2.9 10 3 T( K) = 2.9 106 T( K) m nm Allows us to estimate the temperature of celestial bodies! Read: Chap 5 02/11/16 slide 17

Spectra of elements Kirchoff & Bunsen (c1850): Each chemical element produces a unique set of spectral lines Not a Planck spectrum! Read: Chap 5 02/11/16 slide 18

Kirchoff & Bunsen cataloged spectra of all known elements Observed spectrum of Sun Part of solar spectrum (upper) Iron lines in lab (lower) Pattern matches elements whether dark or light lines Lines found in Sun not known on Earth: discovery of Helium! Read: Chap 5 02/11/16 slide 19

How is energy stored in atoms? Electrons in atoms are restricted to particular energy levels ( quantum mechanics ) Read: Chap 5 02/11/16 slide 20

Internal energy Energy Level Transitions Electron absorbs or emits photon Only allowed changes in energy correspond to a transition between energy levels Read: Chap 5 02/11/16 slide 21

Internal energy Energy Level Transitions Electron absorbs or emits photon Only allowed changes in energy correspond to a transition between energy levels Disallowed! Read: Chap 5 02/11/16 slide 21

Internal energy Energy Level Transitions Electron absorbs or emits photon Only allowed changes in energy correspond to a transition between energy levels Disallowed! Allowed! Read: Chap 5 02/11/16 slide 21

Chemical fingerprints Downward transitions produce a unique pattern of emission lines Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths [demo] Read: Chap 5 02/11/16 slide 22

Chemical fingerprints Each atom has unique spectral figureprint Read: Chap 5 02/11/16 slide 23

1. A dense (opaque) hot object emits a continuous spectrum 2. A cool tenuous cloud emits discrete spectral lines 3. A hot opaque object viewed through a cool cloud shows a continuous spectrum with absorption lines Virtually every astronomical observation fits in one of these categories. Read: Chap 5 02/11/16 slide 24

Read: Chap 5 02/11/16 slide 25

Continuous spectrum The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption Read: Chap 5 02/11/16 slide 26

Example: Read: Chap 5 02/11/16 slide 27

Which letter(s) labels absorption lines? A B C D E Read: Chap 5 02/11/16 slide 28

Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E Read: Chap 5 02/11/16 slide 29

Which letter(s) label emission lines? A B C D E Read: Chap 5 02/11/16 slide 30

How does light tell us the speed of a distant object? Read: Chap 5 02/11/16 slide 31

Measuring the shift Stationary Moving away Away faster Moving toward Toward faster We generally measure the Doppler Effect from shifts in the wavelengths of spectral lines Read: Chap 5 02/11/16 slide 32

Doppler shift tells us ONLY about the part of an objects motion toward or away from us: Read: Chap 5 02/11/16 slide 33

I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. What can I say about this star? (a) It is moving away from me. (b) It is moving toward me. (c) It has unusually long spectral lines. Read: Chap 5 02/11/16 slide 34